1. Field of the Invention
The invention relates to the purification of hydrogen. More particularly, it relates to the purification of hydrogen present in high pressure gas streams containing a relatively high percentage of impurities.
2. Description of the Prior Art
The pressure swing adsorption (PSA) process provides a highly desirable means for separating and purifying hydrogen contained in a feed gas mixture thereof with impurities that are selectively adsorbed by one or more adsorbent beds in a PSA system. Adsorption occurs in such beds at a higher adsorption pressure, with the selectively adsorbable impurities thereafter being desorbed by pressure reduction to a lower desorption pressure. The beds may be purged at said lower pressure for further desorption and removal of impurities, if desired, before repressurization to the higher adsorption pressure for adsorption of impurities from additional quantities of the feed gas mixture as the processing sequence is carried out, on a cyclic basis, in each bed in the adsorption system.
The PSA process is commonly employed in multi-bed systems. The Wagner patent U.S. Pat. No. 3,430,418, discloses a PSA process and system employing at least four adsorbent beds arranged for carrying out a particular PSA processing sequence on a cyclic basis. This sequence includes higher pressure adsorption, cocurrent depressurization to intermediate pressure with release of void space gas from the discharge or product end of the bed, countercurrent depressurization to a lower desorption pressure, and repressurization to a higher adsorption pressure. Wagner teaches the passing of released void space gas from one bed directly to another bed initially at its lower desorption pressure. The pressure in the two beds is thereby equalized at an intermediate pressure. Additional void space gas can be released from the product end of the bed being cocurrently depressurized, with such void space gas being used to provide purge gas to another bed in the PSA system before such other bed is repressurized from its lower desorption pressure. After each bed had been repressurized to an intermediate pressure level by such pressure equalization, it is further repressurized from the intermediate level to its higher adsorption pressure, generally at least in part by the countercurrent addition of product effluent to the product end of the bed being repressurized.
In a further development in the art, the Fuderer patent U.S. Pat. No. 3,986,849, discloses the use of at least seven adsorbent beds, with the feed gas mixture being introduced to the feed end of at least two adsorbent beds, in overlapping identical processing cycles, at all stages of the PSA processing sequence. Each bed, in turn, undergoes three pressure equilization steps prior to repressurization to the higher adsorption pressure, with said steps being carried out in a particular manner achieving higher product purity by substantially avoiding the impurity profile reversion that can occur upon pressure equilization as discussed in the patent. It is also within the contemplation of the art to employ, in some circumstances, a fourth pressure equalization step prior to final repressurization with product effluent.
The PSA process, as represented by the disclosures of the patents, is a highly desirable and practical commercial process for the purification of hydrogen. It has the advantage of being capable of producing very high purity product, e.g. in excess of 99.9 percent pure hydrogen product. The PSA process can be used to treat a wide range of available feedstocks, and is not limited to use with a particular hydrogen-containing feed gas stream. No pretreatment or post-treatment steps are required as part of the process, other than such conventional impurity removal as may be desirable or required to avoid undue degradation of the adsorbent. In addition, there is essentially no pressure drop between the feed gas stream and the product gas so that the product gas is available at the adsorption pressure level for further use downstream of the PSA system and for repressurization of each bed to said adsorption pressure from a lower desorption level or from the intermediate pressure level achieved during one or more pressure equalization steps carried out in preferred embodiments of the invention.
While the PSA process is thus highly desirable for use in commercial applications, there are certain unfavorable aspects encountered in applying the process at very high adsorption pressures, e.g. above about 600 psig. At such high pressures, the PSA system for carrying out the process becomes relatively expensive in terms of capital investment costs. In addition, there is found to be a potentially lower recovery of product gas at such high pressure levels. Thus, more of the desired product gas tends to be stored in the bed at such high adsorption pressure levels, with said gas being discharged from the bed with the impurities during the countercurrent depressurization step. As a result, there is a desire in the art to improve the overall performance of PSA systems at pressures above about 600 psig. Such improvements would advantageously increase product recovery and reduce the overall cost of operation so as to enhance the overall technical and economic feasibility of employing the PSA process and system as in the separation and purification of hydrogen from hydrogen-containing feedstocks or off-gases available at high pressures, particularly such gas having a high concentration of impurities.
It is an object of the invention, therefore, to provide an improved overall PSA process and system for the treatment of high pressure gas streams.
It is another object of the invention to provide a PSA process and apparatus for improving the separation and recovery of hydrogen product gas from feed gas streams available at high pressures and having a high concentration of impurities.
It is a further object of the invention to provide a process and apparatus for reducing the PSA capital investment costs associated with the purification of high pressure hydrogen-containing gas streams containing a relatively large amount of impurities.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.